1. Field of the Invention
The present invention relates to a system for purifying exhaust gases from an internal combustion engine and, more particularly, to an engine exhaust gas purification system of the type in which an air-fuel ratio sensor is disposed in the exhaust system of the engine to detect the oxygen content of the engine exhaust gases and to emit an output signal on the basis of which the supply of a secondary air into the engine exhaust system is controlled to maintain a substantially constant air-fuel ratio in the exhaust system so that a three way catalyst is most effectively operative to facilitate removal of the harmful components of the engine exhaust gases.
2. Description of the Prior Art
In the engine exhaust gas purification system of the type specified above, the secondary air supply is controlled by a metering valve which is operative in response to signals from the air-fuel ratio sensor. The applicants, however, have found through researches that, in spite of the secondary air supply being controlled in the manner discussed, a problem of overheating takes place in the engine exhaust system in a high load engine operating condition. It has also been known that the exhaust gases flow back into the secondary air supply system due to a high exhaust gas pressure or back pressure produced in a high load engine operating condition.
The secondary air metering valve can be used to interrupt the supply of the secondary air into the engine exhaust system in a high load engine operating condition. In this case, however, the spring for the metering valve must provide a predetermined spring force which is so small as to assure a good response of the metering valve. Such a small spring force would, in some cases, be insufficient to enable the metering valve to surely block the secondary air supply passage against a counter-flow of the engine exhaust gases during a high load engine operation.